Emission reduction method for use with a heat recovery steam generation system

Abstract

A system and method of maintaining an optimal temperature range for a catalyst section in a HRSG comprising placing a portion of the exhaust stream in a heat exchanger and superheater, diverting a second portion around the heat exchanger and superheater, combining the two portions and contacting the two portions with a catalyst section. Alternatively, a system of heat exchangers are employed to address the fluctuating exhaust temperature caused by the intermittent use of the duct burners.

1. A method of maintaining an optimal temperature for a catalyst section in a heat recovery steam generation system comprising:
 Placing a first portion of a gas turbine exhaust stream in a path of a first heat exchanger; 
 Diverting at least one second portion of the exhaust stream to avoid the first heat exchanger, wherein the at least one second portion combines with the first portion after the first heat exchanger; and 
 Contacting the combined first and second portions with a catalyst section, wherein the catalyst section comprises one or more reducing agent injection points, one or more SCR catalysts, and one or more CO catalysts disposed upstream of said one or more SCR catalysts and downstream of said one or more reducing agent injection points. 
 
     
     
       2. The method of  claim 1  wherein the catalyst section has a temperature of about 715° F. to about 815° F., and wherein a temperature of said at least one second portion is greater than a temperature of said combined first and second portions. 
     
     
       3. The method of  claim 1  further comprising directing the exhaust stream through one or more duct burners prior to said diverting. 
     
     
       4. The method of  claim 1  wherein the diverting includes diverting said at least one second portion to avoid contacting said at least one second portion with one or more superheaters. 
     
     
       5. The method of  claim 1  wherein the contacting comprises
 Injecting a reducing agent into the combined portions via the injection points; and 
 Flowing the combined portions into one or more SCR catalyst, and 
 
       wherein the SCR catalyst is one or more of a HC-SCR, ethanol-SCR or ammonia-SCR catalyst and the reducing agent comprises one or more hydrocarbon, ethanol, ammonia, aqueous ammonia, an ammonia generating compound, urea and combinations thereof. 
     
     
       6. The method of  claim 1  further comprising contacting said combined first and second portions with a second heat exchanger disposed downstream of the catalyst section. 
     
     
       7. The method of  claim 2  wherein the first portion of said exhaust gas prior to said first heat exchanger has a temperature of about 825° F. to about 1100° F. 
     
     
       8. The method of  claim 1  wherein the SCR catalyst is a zeolite-based catalyst. 
     
     
       9. A method of maintaining an optimal temperature for a catalyst section in a heat recovery steam generation system comprising:
 Placing a first portion of a gas turbine exhaust stream in a path of a first heat exchanger; 
 Diverting at least one second portion of the exhaust stream to avoid the first heat exchanger and to avoid contacting said at least one second portion with one or more superheaters, wherein the at least one second portion combines with the first portion after the first heat exchanger; and 
 Contacting the combined first and second portions with a catalyst section, wherein the catalyst section comprises one or more reducing agent injection points and one or more SCR catalysts. 
 
     
     
       10. The method of  claim 9  wherein the catalyst section has a temperature of about 715° F. to about 815° F., and wherein a temperature of said at least one second portion is greater than a temperature of said combined first and second portions. 
     
     
       11. The method of  claim 10  wherein the first portion of said exhaust gas prior to said first heat exchanger has a temperature of about 825° F. to about 1100° F. 
     
     
       12. The method of  claim 9  further comprising directing the exhaust stream through one or more duct burners prior to said diverting. 
     
     
       13. The method of  claim 12  wherein said one or more duct burners operate intermittently to produce an optimal temperature in the catalyst section. 
     
     
       14. The method of  claim 9  wherein the contacting comprises
 Injecting a reducing agent into the combined portions via the injection points; and 
 Flowing the combined portions into one or more SCR catalyst, and 
 
       wherein the SCR catalyst is one or more of a HC-SCR, ethanol-SCR or ammonia-SCR catalyst and the reducing agent comprises one or more hydrocarbon, ethanol, ammonia, aqueous ammonia, an ammonia generating compound, urea and combinations thereof. 
     
     
       15. The method of  claim 9  further comprising contacting said combined first and second portions with a second heat exchanger disposed downstream of the catalyst section. 
     
     
       16. The method of  claim 9  wherein the catalyst section further comprises one or more CO catalysts disposed downstream of said one or more SCR catalysts. 
     
     
       17. The method of  claim 16  wherein the contacting comprises
 Injecting a reducing agent into the combined portions via the injection points; and 
 Flowing the combined portions into one or more SCR catalysts. 
 
     
     
       18. The method of  claim 17  wherein the placing step further comprises placing the first portion in a path of the one or more superheaters. 
     
     
       19. The method of  claim 17  further comprising directing the exhaust stream through one or more duct burners first. 
     
     
       20. The method of  claim 9  wherein the SCR catalyst is a zeolite-based catalyst.